The present invention relates to equipment for connection to an alternating current network for power factor correction.
It is desirable to achieve equipment in which the reactance can be controlled in a simple and rapid manner. A typical example of a field of use of such a device is power factor correction, i.e., correction of the reactive (usually inductive) power consume by one or more load objects connected to a power supply system, so that the resulting reactive power consumption becomes low and preferably as close to zero as possible.
For this purpose, one technique is to use a number of capacitor banks, each of which can be selectively connected or disconnected from the network depending on the need for correction. It is also known to use a capacitor bank which is permanently connected to the network, together with a number of reactors, each of which can be selectively connected to or disconnected from the network. In this case, for example, the sum of the rated powers of the reactors may be equal to the rated power of the capacitor bank, the reactors being successively disconnected when the need for compensating capacitive reactance increases.
In both these cases the reactance of the equipment can only be varied in discrete steps, and for practical and economical reasons, the number of steps must be relatively low. Since the need for correction as a rule varies continuously and randomly, this means that the correction is usually never exact but that a certain uncorrected reactive power necessarily remains. Since the number of steps must be low, the average residual uncorrected power will be so great that it constitutes a considerable disadvantage in the known equipment mentioned.
It is further known to use a fixed capacitor bank and a reactor in series with a pair of parallel, oppositely poled thyristors, connected to the network. By phase angle control of the thyristors, the current through the reactor can be controlled continuously. If, for example, the rated powers of the reactor and the capacitor bank are equal, the reactance of the equipment can be controlled, in principle, continuously between zero and the reactance of the capacitor bank. However, by the phase angle control, overtones occur in a known manner in the load current of the equipment, and the current overtones cause voltage overtones in the network because of the network impedance. As is clear from the above, the rated power of the reactor is of the same order of magnitude as the maximum need for correction, and this means that current and voltage overtones will be of such magnitude to constitute considerable disadvantage in this type of equipment.
The invention aims to provide a simple and economically advantageous equipment of the kind mentioned in the introduction, the reactance of which can be continuously controlled in a simple manner within a great range and which causes a minimum of current and voltage overtones.